1. Field of the Invention
The present invention relates to a piezoelectric element and a method of manufacturing the piezoelectric element, and more particularly, the present invention relates to a bimorph piezoelectric element which may be used in forming an acceleration sensor and a method of manufacturing the piezoelectric element.
2. Description of the Background Art
In general, an acceleration sensor including a piezoelectric element may be used for detecting impact or the like. With reference to FIG. 1, one example of a piezoelectric element 1 will be described.
The piezoelectric element 1 has a piezoelectric ceramic body 2. First, second and third surface electrodes 3, 4 and 5, respectively are formed on an upper surface of the piezoelectric ceramic body 2 by a thin film forming method such as sputtering. The electrodes 3, 4, and 5 are space at longitudinal intervals along the piezoelectric ceramic body 2. Further, a first connecting electrode 6 is formed on the upper surface of the piezoelectric ceramic body 2, for electrically connecting the first to third surface electrodes 3 to 5 with each other. A first signal drawing electrode is formed by the first to third surface electrodes 3 to 5 and the first connecting electrode 6.
On the other hand, first, second and third surface electrodes 7, 8 and 9 are formed on a lower surface of the piezoelectric ceramic body 2 in first to third portions thereof respectively, by a thin film forming method. A second connecting electrode 10 is formed to at least partially cover the first to third surface electrodes 7 to 9, for electrically connecting the electrodes with each other. A second signal drawing electrode is formed by the first to third surface electrodes 7 to 9 and the second connecting electrode 10.
The piezoelectric ceramic body 2 has a longitudinally extending internal electrode 11 located at an intermediate vertical position. As shown in FIG. 1, the internal electrode 11 does not extend to either longitudinal end of the piezoelectric ceramic body 2.
The interior of the piezoelectric ceramic body 2 is polarized in a manner described in the following paragraph. In a piezoelectric ceramic body region 2A which is located above the portion provided with the internal electrode 11, the second portion is downwardly polarized as shown by arrow B while the first and third portions are upwardly polarized as shown by arrows A and C, respectively. In a piezoelectric ceramic body region 2B which is located under the internal electrode 11, the first to third portions are polarized oppositely to the polarization directions of the piezoelectric ceramic body region 2A which is located above the internal electrode 11, as shown by arrows D, E and F, respectively. In other words, the upper and lower piezoelectric ceramic body regions 2A and 2B are polarized in opposite directions in each of the first to third portions. In each of the respective piezoelectric ceramic body regions 2A and 2B, the second portion and the first and third portions are polarized in opposite directions.
On the upper surface of the piezoelectric ceramic body 2, the first surface electrode 3 extends to a side edge of the piezoelectric ceramic body 2, whereby an end of the first signal drawing electrode which is located on the upper surface extends to this side surface of the piezoelectric ceramic body 2. On the lower surface of the piezoelectric ceramic body 2, the third surface electrode 9 is also formed to extend to another side edge of the piezoelectric ceramic body 2, whereby the second signal drawing electrode which is located on the lower surface extends to this side edge of the piezoelectric ceramic body 2.
Broken lines G and H show the respective boundaries between the first to third portions of the piezoelectric ceramic body 2. The first, second and third portions are located on the left side of the boundary G, between the boundaries G and H, and on the right side of the boundary H, respectively.
The piezoelectric ceramic body 2 is held by frame bodies 12 and 13 which are arranged on respective upper and lower portions thereof. Each of the frame bodies 12 and 13 is made of insulating ceramics such as alumina or another material having a desired rigidity, and has a flat plate portion and a pair of fixed portions extending toward the piezoelectric ceramic body 2 from both ends of the flat plate portion. The frame body 12 is fixed to the upper surface of the piezoelectric ceramic body 2 at forward ends of the pair of fixed portions. Similarly, the frame body 13 is fixed to the lower surface of the piezoelectric ceramic body 2 at forward ends of the pair of fixed portions.
The piezoelectric element 1 has such a structure such that the frame bodies 12 and 13 are fixed to the upper and lower portions of the piezoelectric ceramic body 2. External electrodes 14 and 15 are located on both side surfaces of this structure. The external electrode 14 is electrically connected to the signal drawing electrode which is located on the upper surface of the piezoelectric body 2, i.e., the first surface electrode 3. On the other hand, the external electrode 15 is electrically connected to the signal drawing electrode which is located on the lower surface of the piezoelectric ceramic body 2, i.e., the third surface electrode 9.
When this piezoelectric element 1 is provided in an acceleration sensor, the piezoelectric element 1 functions as described below. When acceleration acts on the piezoelectric element 1, respective central portions of the piezoelectric ceramic body regions 2A and 2B forming the piezoelectric ceramic body 2, i.e., the second portions, and the first and third portions are deformed in opposite directions by inertial force. In this case, the second portions and the first and third portions are subjected to a tensile force or a compressive stress resulting from the aforementioned deformation. When the central second portions are subjected to tensile stress, for example, the first and third portions are subjected to compressive stress. Because the second portions and the first and third portions are polarized in opposite directions, the quantity of electric charges generated in the entire piezoelectric ceramic body 2 is greatly increased by electric charges generated by the stress in the second portions and the first and third portions. Thus, it is possible to form an acceleration sensor having excellent detection sensitivity.
A method of manufacturing the piezoelectric element 1 shown in FIG. 1 is now described with reference to FIGS. 2A to 2C and 3A and 3B. This method is adapted to form the piezoelectric element 1 shown in FIG. 1 from a mother piezoelectric ceramic body material. Areas corresponding to individual piezoelectric elements are divided by phantom lines X, Y and Z in FIGS. 2A to 2C and 3A and 3B.
First, a mother piezoelectric ceramic body 16 which is in the form of an elongated plate is prepared as shown in FIG. 2A. Internal electrodes 11 are formed in the piezoelectric ceramic body 16 at an intermediate vertical position, to extend in the longitudinal direction. While a plurality of internal electrodes 11 are formed in FIG. 2A, one internal electrode 11 is provided in the resulting piezoelectric element 1 shown in FIG. 1.
The mother piezoelectric ceramic body 16 is divided into upper and lower piezoelectric ceramic body regions 16A and 16B through the portion provided with the aforementioned internal electrodes 11.
On an upper surface of the piezoelectric ceramic body 16, a plurality of sets of first to third surface electrodes 3 to 5 are formed so as to extend longitudinally along the piezoelectric ceramic body 16.
On a lower surface of the piezoelectric ceramic body 16, a plurality of sets of first to third surface electrodes 7 to 9 are similarly formed so as to extend in the longitudinal direction. The first to third surface electrodes 3 to 5 and 7 to 9 are formed to be positioned in the aforementioned first to third portions, respectively.
Then, polarization is carried out through the internal electrodes 11 and the first to third surface electrodes 3 to 5 and 7 to 9. Namely, relatively high voltages, relatively low voltages, and intermediate voltages are applied to the second surface electrodes 4 and 8, the first and third surface electrodes 3, 5, 7 and 9, and the internal electrodes 11, respectively, thereby polarizing the respective piezoelectric ceramic body regions 16A and 16B as shown by arrows A to C and D to F in FIG. 2B.
Then, first and second connecting electrodes 6 and 10 are located on the first to third surface electrodes 3 to 5 and 7 to 9, respectively in the individual piezoelectric element portions, as shown in FIG. 2C.
Then, mother frame bodies 17 and 18 are connected to and integrated with upper and lower portions of the piezoelectric ceramic body 16, respectively by adhesives, as shown in FIG. 3A. Further, the structure shown in FIG. 3A is cut along two-dot chain lines X, Y and Z to obtain individual piezoelectric elements 1, thereby obtaining a structural body 19 shown in FIG. 3B. The signal drawing electrode which is located on the upper surface of the piezoelectric ceramic body 2, i.e., the first surface electrode 3, is exposed on a first side surface of the structural body 19 obtained in the aforementioned manner. Similarly, an end of another signal drawing electrode, i.e., the third surface electrode 9, is exposed on a second side surface of the piezoelectric ceramic body 2. The external electrodes 14 and 15 shown in FIG. 1 are formed on these side surfaces, so that the respective signal drawing electrodes are electrically connected with the external electrodes 14 and 15 for obtaining the piezoelectric element 1.
However, the aforementioned method of manufacturing the piezoelectric element 1 has the following problems. The thicknesses of the first to third mother surface electrodes 3 to 5 and 7 to 9 which are formed on the upper and lower surfaces of the mother piezoelectric ceramic body 16 may be reduced depending on the forming conditions. The reduced thicknesses of the surface electrodes 3 to 5 and 7 to 9 may destroy the electrical connection between the external electrodes 14 and 15 and the surface electrodes 3 and 9, i.e., the electrical connection between the external electrodes 14 and 15 and the respective signal drawing electrodes.
In addition, when the connecting electrodes 6 and 10 are formed by screen-printing conductive paste and baking the paste depolarization is caused in the piezoelectric ceramic body 16 by heat which is applied during baking. When slight depolarization is caused, detection sensitivity is reduced in the resulting acceleration sensor. Thus, the non-defective rate as and mass productivity of the acceleration sensor are disadvantageously reduced by the conventional method.